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Strange Moonlight

10.03.06

When the Harvest Moon rises on Oct. 6th, go outside. You may notice a few puzzling things.

Not so long ago, before electric lights, farmers relied on moonlight to
harvest autumn crops. With everything ripening at once, there was too much work
to to do to stop at sundown. A bright full moon--a "Harvest Moon"--allowed work
to continue into the night.

The moonlight was welcome but, as any farmer could tell you, it was strange
stuff. How so? See for yourself. The Harvest Moon of 2006 rises on October 6th,
and if you pay attention, you may notice a few puzzling things:

1. Moonlight steals color from whatever it touches. Regard a rose. In
full moonlight, the flower is brightly lit and even casts a shadow, but the red
is gone, replaced by shades of gray. In fact, the whole landscape is that way.
It's a bit like seeing the world through an old black and white TV set.

"Moon gardens" turn this 1950s-quality of moonlight to advantage. White or
silver flowers that bloom at night are both fragrant and vivid beneath a full
moon. Favorites include Four-O'clocks, Moonflower Vines, Angel's Trumpets--but
seldom red roses.

2. If you stare at the gray landscape long enough, it turns blue. The
best place to see this effect, called the "blueshift" or "Purkinje shift" after
the 19th century scientist Johannes Purkinje who first described it, is in the
countryside far from artificial lights. As your eyes become maximally dark
adapted, the blue appears. Film producers often put a blue filter over the lens
when filming night scenes to create a more natural feel, and artists add blue
to paintings of nightscapes for the same reason. Yet if you look up at the full
moon, it is certainly not blue. (Note: Fine ash from volcanoes or forest fires
can turn moons blue, but that's another story.)

3. Moonlight won't let you read. Open a book beneath the full moon.
At first glance, the page seems bright enough. Yet when you try to make out the
words, you can't. Moreover, if you stare too long at a word it might fade away.
Moonlight not only blurs your vision but also makes a little blind spot.
(Another note: As with all things human, there are exceptions. Some people have
extra-sensitive cones or an extra helping of rods that do allow them to read in
the brightest moonlight.)

This is all very strange. Moonlight, remember, is no more exotic than
sunlight reflected from the dusty surface of the moon. The only difference is
intensity: Moonlight is about 400,000 times fainter than direct sunlight.

So what do we make of it all? The answer lies in the eye of the beholder.
The human retina is responsible.

The retina is like an organic digital camera with two kinds of pixels: rods
and cones. Cones allow us to see colors (red roses) and fine details (words in
a book), but they only work in bright light. After sunset, the rods take
over.

Rods are marvelously sensitive (1000 times more so than cones) and are
responsible for our night vision. According to some reports, rods can detect as
little as a single photon of light! There's only one drawback: rods are
colorblind. Roses at night thus appear gray.

If rods are so sensitive, why can't we use them to read by moonlight? The
problem is, rods are almost completely absent from a central patch of retina
called the fovea, which the brain uses for reading. The fovea is densely packed
with cones, so we can read during the day. At night, however, the fovea becomes
a blind spot. The remaining peripheral vision isn't sharp enough to make out
individual letters and words.

Finally, we come to the blueshift. Consider this passage from a 2004 issue
of the Journal of Vision:

"It should be noted that the perception of blue color or any color for that
matter in a purely moonlit environment is surprising, considering that the
light intensity is below the detection threshold for cone cells. Therefore if
the cones are not being stimulated how do we perceive the blueness? --"Modeling
Blueshift in Moonlit Scenes using Rod-Cone Interaction" by Saad M. Khan and
Sumanta N. Pattanaik, University of Central Florida.

The authors of the study went on to propose a bio-electrical
explanation--that signals from rods can spill into adjacent blue-sensitive
cones under conditions of full-moon illumination (see the diagram, right). This
would create an illusion of blue. "Unfortunately," they point out, "direct
physiological evidence to support or negate the hypothesis is not yet
available."

So there are still some mysteries in the moonlight. Look for them on Oct.
6th under the Harvest Moon.

Caveat Lunar: This story makes some generalizations about what
people can see at night but, as with all things human, there are exceptions:
Some people can read by moonlight; others have no trouble seeing the red petals
of a moonlit rose. These people have "moonvision," boosted by an extra-helping
of rods or unusually sensitive cones. Are you one of them?

More Information

More information about the moonlight blueshift:

The blueshift is sometimes attributed to the spectral response of rods.
Although rods are nominally color blind, they do not respond equally to all
colors: Rods are more sensitive to blue-green photons and less sensitive to red
photons. You can see this in your moonlit rose. By day, the red flower
dominates the green leaves. At night, the situation is reversed. The green
leaves are more vivid than the red flower.

No matter which part of the rose stands out most, however, the ensemble is
still gray. This is because the rods have no mechanism for separating colors.
Shades of gray are all we get.

Cones are able to separate colors because they come in three varieties:
red-sensitive, green-sensitive, and blue-sensitive. The brain can sort out the
color of an object by noting which kind of cone it stimulates most.

Rods, on the other hand, come in one variety only--monochromatic, which
brings us back to the mystery of the blue shift. If rods can't separate colors,
how does the brain register a blue rather than gray landscape? Khan and
Pattanaik's hypothesis of rod signals "bleeding" into adjacent blue-sensitive
cones provides a possible but untested explanation.

The Eye and Night Vision --
from the USAF Special Report, AL-SR-1992-0002, "Night Vision Manual for the
Flight Surgeon", written by Robert E. Miller II, Col, USAF, (RET) and Thomas J.
Tredici, Col, USAF, (RET)